As is well known in electrical energy distribution systems, switchgear panels are electrical components used as node points, while cables and overhead lines represent the conjunction of the various nodes.
From a structural point of view, known switchgear panels, which can be also indicated with the equivalent terms of electric switchboards, or simply switchgear or electric panels, or similar definitions, usually comprise a metallic enclosure that is internally divided into several compartments or cells housing various apparatuses and equipment. For example, one compartment houses a switching unit, such as a circuit breaker; a second compartment houses main cables, such as bus-bars, feeding power from an electrical source; a further compartment houses a system of cables suitable to be connected to a load, for example an electrical motor. Depending on the application, switchgear panels may comprise other components that include but are not limited to current transformers, fuses, and voltage transformers.
During the working life of a switchgear panel, electrical faults or malfunctions may occur, such as short circuits, current overloads, and in particular, arcing events. Arcing events, e.g. arcing faults or flashes, occur when electric current arcs strike between two conductors inside the switchgear cabinet, e.g. between phase conductors, phase and neutral conductors, between the contacts of the circuit breaker used, between a conductor and ground, or another situation. When arcing faults occur, in particular in medium- to high-voltage power applications wherein the levels of energy involved are quite significant, the ionized gas associated with them may be released at significant pressures and temperatures sufficient to severely damage or destroy the switchgear panel, and/or the devices and equipment housed inside. An arcing fault may also be dangerous for operating personnel or equipment outside the enclosure. In particular, the materials involved in or exposed to the arc produce hot decomposition products, both gaseous and particulate either plastic and/or metallic, which may be discharged to the outside of the enclosure together with or in addition to hot gases and flames.
Due to the aforementioned problems, safety standards and related tests have been introduced. For example, one typical standard test utilizes highly flammable indicators, such as pieces of cotton or equivalent material, which are placed outside the switchgear enclosure around the perimeter of the enclosure at a certain distance from some or all of the lateral walls of the enclosure. During the test and more particularly, when an internal arc is caused to strike, these flammable indicators must not ignite.
Hence, such standards have imposed onto switchgear manufacturers the adoption of protection systems suitable to prevent and/or mitigate the effect of possible electrical arcs. In particular, many known solutions are focused on early detection of an occurring electric arc with a consequent fast intervention devoted to prevent or mitigate the undesired effects of an electric arc at an early stage.
For example, a first solution foresees the evaluation of current perturbations in a conductor which are indicative of an arcing event; however, this solution may require onerous processing demands resulting in an undesirably long reaction time for identifying an arcing event. Another solution uses pressure sensors to monitor the increase in pressure indicative of an arcing event; also this solution may require significant time before pressure increases to detectable levels, resulting in long reaction times before mitigating an arcing event.
One of the most used solutions for detecting arcing events involves the use of optical detectors, such as optical fibers, to detect visible light and thereby to sense the arc flash associated with an arcing event. However, this solution may result in erroneous detections as the light sensors may detect light from sources independent from electric arcs. This may further result in unwarranted tripping of protection units which would put out of service the whole switchgear panel or parts thereof.
In some cases, a combination of different types of arc protection systems has been used. For example, undesired interventions have been partially limited by combining the detection of light possibly linked to an occurring electric arc with monitoring of the current levels flowing into the part of the electric system being monitored.
Other solutions are instead focused on limiting the resulting effects of an electric arc. For example, arc-resistant switchgear cabinets are widely used and able to mechanically withstand the pressure waves and temperatures of the hot gases associated with an arcing fault; in some other cases there are provided specific parts of the enclosure which may blow up due to an electric arc occurring inside the enclosure.
In addition or in alternative, there are provided also suitable paths for channeling and venting the hot gases and flames generated by an arcing fault out from the internal compartments towards a desired area. For example, there are provided flaps or ducts which lead the hot gases, flames and particulate to the outside environment through the ceiling of the enclosure or in some cases even underground, i.e. toward zones far away from the possible presence of operating personnel.
Although known solutions perform satisfactorily, there is still room for further improvements in arc protection for switchgear. For example, the ceiling and wall of the room wherein the switchgear panel is located may deflect hot gasses, flames and particles, down onto personnel working near the panel thus being still potentially dangerous.